1. Field of the Invention
This invention relates generally to a transistor amplifier, and more particularly is directed to a transistor amplifier in which field effect transistors are employed as amplifying devices in all of its amplifying stages.
2. Description of the Prior Art
In the well known field effect transistor which will be hereinafter referred to as an FET, the channel in which the drain current I.sub.D flows generally is parallel to its semiconductor substrate or in its lateral direction and the current flowing through this channel is controlled by supplying a reverse biased voltage V.sub.GS to its gate. When the drain voltage arrives at a certain value, the current is saturated (V.sub.DS -I.sub.D characteristics) and so called pentode characteristics are obtained. Since the transmission characteristics (V.sub.GS -I.sub.D characteristics) are square characteristics, such pentode characteristic FETs have relatively poor distortion characteristics.
FETs with triode characteristics are known in which a current flows perpendicular to the substrate or in its longitudinal direction in a manner similar to that in a bipolar transistor. This type of FET can be made to have a high current utility factor per unit area and provides triode characteristics with a low output resistance. Accordingly, when the above triode characteristic FET is used in a power amplifier for audio use, the quality of the sound obtained is similar to that achieved when a vacuum triode tube is used. Further, when a triode characteristic FET is compared with a vacuum triode tube, the output resistance of the former is much lower than that of the latter, problems due to the heat generated by a heater of a vacuum tube are avoided, and elements with complementary characteristics can be obtained. Thus, if triode characteristic FETs are used in the last stage of an audio power amplifier circuit, a pure complementary OTL circuit can be easily realized. Further, when compared with a bipolar transistor, the triode characteristic FET has a relatively wide safe operation range and easily produces a relatively great output.
A transistor amplifier employing triode characteristic FETs has already been proposed, for example, as disclosed in U.S. patent application Ser. No. 508,836, filed Sept. 24, 1974, now U.S. Pat. No. 3,921,089. However, such previously proposed transistor amplifier has FETs only in its output or final stage, and thus does not fully realize the advantages of employing such elements.